It is common practice in the prior art to produce toothed wheels or other workpieces having a gearing on special gearing-forming machine tools, i.e. on special single-purpose machines. For example, special gear hobbing machines are known on which a workpiece is machined so as to be provided with a tooth profile by guiding a profile hob or a profile grinding wheel such that the workpiece is machined so as to produce tooth flanks or teeth. Here, the hobs or profile grinding wheels already have a cross-sectional shape in accordance with the desired profile shape of the tooth flanks to be produced with which the workpiece is to be provided. One or two opposite tooth flanks, which have the profile according to the shape of the tool, are formed by guiding the gear hob or the profile grinding wheel along a path through the workpiece.
If a toothed wheel having a herringbone gearing, in particular a toothed wheel having left-hand and right-hand helical gearings (also referred to as double helical gearing) on such single-purpose machines by means of profile grinding wheels or profile hobs, it is first necessary to machine one or a plurality of tooth profiles of one helical gearing in a first processing step, and then one or a plurality of tooth profiles of the other helical gearing having opposite pitch can be machined in another processing step. This results in an expensive and time-consuming manufacturing process of the toothed wheel having the herringbone gearing. Moreover, the teeth of the herringbone gearing cannot be machined continuously in the conventional processes on single-purpose machines using a profile hob or a profile grinding wheel as when a tooth of one helical gearing is machined by means of a profile hob or a profile grinding wheel up to the middle of the herringbone gearing, this tool will already cut or grind into a tooth flank of a tooth of the other helical gearing. For this reason, a workpiece having a central groove that separates the two helical gearings of the herringbone gearing is used for the production of a toothed wheel having a herringbone gearing by means of a profile hob or a profile grinding wheel.
This is illustrated by way of example in FIG. 1, which shows a perspective view of a workpiece 10 having a central groove 11 circumferential around the rotationally symmetrical workpiece. The workpiece 10 also already has a space width 12 of one helical gearing and a space width 13 of the other helical gearing. The opposite tooth flanks 12a and 12b of the space width 12 are separated from the tooth flanks 13a and 13b of the space width 13 by the central groove 11. This prevents a profile hob or a profile grinding wheel from cutting or grinding into a tooth flank of the respectively other space width when the space widths 12 are machined. FIG. 2 shows a detail of space widths 12 and 13 of FIG. 1 by way of example. FIG. 3 shows a top view onto space widths 12 and 13 of FIG. 1 by way of example. As shown in FIG. 3, the two helical gearings of the herringbone gearing have opposed pitches and equal helix angles. Groove 11 has a groove width NB. The finished toothed wheel thus has teeth of the two helical gearings of the herringbone gearing which in each case are separated in the tip of the herringbone gearing by groove 11 having groove width NB. However, when two toothed wheels roll, the material is exposed to high loads and stresses, especially on the outer sides of flanks 12a, 12b, 13a and 13b on the outer surface and in particular on the edges of the tooth flanks 12a, 12b, 13a and 13b which face groove 11. This reduces the loadability of the toothed wheel during rolling with a toothed wheel having mating flanks under load.
However, the present invention relates to a newly developed method of producing workpieces having a gearing on numerically controlled universal machine tools comprising at least 5 axes, such as a milling machine, a universal milling machine or a machining center, in which the gearing is produced on the workpiece by successively moving standard milling cutters, such as an end mill with or without end radius, in a line-by-line process along machining paths so as to cut a tooth flank out of the workpiece. In contrast to the production of workpieces having a gearing on the above known, conventionally used gearing-forming single-purpose machines, the object of the invention is to provide a workpiece with a gearing on universal machine tools, such as a milling machine, a universal milling machine or a machining center.
Such a production of a workpiece having a gearing on a universal machine tool is described in the Article “Auf einfachem Weg zu guten Zähnen-Zahnräder mit hoher Qualität auf Standardmaschinen fräsen” by Hans-Peter Schossig (published in the journal WERKSTATT UND BETRIEB, Carl Hanser Verlag, Munich, issue 2007, no. 4/28, pages 28-32, ISSN 0043-2792) and WO 2008/133517 A1, for example. The present invention is based on this newly developed method and its further developments developed by Applicant itself (see e.g. DE 10 2009 008 124).
It has been discovered with respect to the above described drawbacks regarding the production of a toothed wheel having a herringbone gearing on single-purpose machines by means of a profile hob or a profile grinding wheel that the loadability of a toothed wheel having a herringbone gearing can be considerably improved when the teeth of one helical gearing are not separated from the teeth of the other helical gearing by a central groove but when the workpiece is continuously machined so as to provide it with teeth. It has also been discovered that this can be achieved by the newly developed method, i.e. producing tooth flanks by means of a milling cutter, in particular an end mill, on a universal machine, such as a milling machine, a universal milling machine or a machining center, as opposed to processes on single-purpose machines having profile hobs or profile grinding wheels. FIG. 4 shows a photographic illustration of a toothed wheel having a herringbone gearing according to a first attempt of producing a workpiece having a herringbone gearing on a universal machine tool by means of an end mill according to a process in accordance with the newly developed method. FIG. 5 shows a photographic detail of the toothed wheel of FIG. 4.
The toothed wheel of FIGS. 4 and 5 no longer has a central groove; the teeth of the two helical gearings of the herringbone gearing are continuously shaped over the entire tooth width of the herringbone gearing and connected with one another in the central tip of the herringbone gearing over the entire depth of teeth, thus considerably improving the loadability of the herringbone gearing in contrast to a toothed wheel having a herringbone gearing and a central groove. However, the tooth flanks 12a and 13a or 12b and 13b of the space widths 12 and 13 of both helical gearings are separated by a central milled recess 11′ so that there is no rolling surface with a mating flank of a mating toothed wheel in the central tip of the herringbone gearing. The tooth flanks are thus prevented from converging or merging into one another at an acute angle in the center region, which might impair the rolling characteristics.
The tooth flanks 12a, 12b, 13a and 13b were machined separately according to a process as known from the Article “Auf einfachem Weg zu guten Zähnen-Zahnräder mit hoher Qualität auf Standardmaschinen fräsen” by Hans-Peter Schossig. The tooth flanks 12a, 12b, 13a or 13b were thus machined separately by successively moving an end mill along machining paths extending at different profile heights in the flank line direction. Correspondingly, FIG. 5 shows milling tracks in the flank line direction in the area of the tooth base between the tooth flanks 12a and 13a or 12b and 13b. The central recess 11′ was then cut by means of an end mill by guiding the end mill along machining paths extending in the profile direction as can be seen in FIG. 5 by means of the milling tracks in the area of recess 11′. However, this process is also expensive and calls for a long machining time since the tooth flanks are machined separately, followed by machining the recess 11′ using a milling cutter having a small tool size and thus a small cutting volume. In addition, high loads occur in the boundary regions of tooth flanks 12a, 12b, 13a and 13b, which border on recess 11′, since no rolling area is available in the central tip of the herringbone gearing.